The present invention relates to a method for compensation of information losses, which have been caused by blanking out pulse-shaped interferences, in a communication signal received by a receiver. Particularly, the invention relates to the application of said compensation method in communication signals on the aviation sector, namely in systems based on “Orthogonal Frequency-Division Multiplexing” (OFDM).
The number of frequency bands available for aeronautical communication is limited so that the increasing demand for communication makes it necessary to search for new solutions. One possible approach is seen in using that range of the frequency bands which serves, inter alia, for operation of ground-based systems provided for distance and/or position determination of aircraft, e.g. the so-called “Distance Measuring Equipment” (DME) or the “Tactical Air Navigation System” (TACAN) which is also employed for military purposes. These systems use the aeronautical range of the L-band between 960 and 1.215 MHz which is subdivided into frequency channels of a respective width of 1 MHz, each of said channels serving for transmission of pulse-shaped signals with high amplitudes. Operated in this frequency range are also military radio systems for data transmission, such as e.g. the so-called “Joint Tactical Information Distribution System” (JTIDS) or the “Multifunctional Information Distribution System” (MIDS). In these systems, frequency channels of a width of respectively 3 MHz are used according to the frequency hopping method; thus, said systems will again generate short pulse-shaped signals on each channel.
The frequency range in which already DME and other systems are operated, is now also to serve for transmitting, parallel to the pulse signals, communication signals which use center frequencies with no or only small frequency offsets to the center frequencies of the pulse signals. For communication, it is provided to use so-called OFDM-based signals. Signals of this type are basically known and are characterized by a sequence of subcarrier signals whose frequencies differ from each other by a constant amount, each subcarrier signal having substantially no signal portions at the frequency of the other subcarrier signals so that substantially no inter-carrier interference (ICI) will occur; see also [1].
However, in the approach according to the invention, a problem would reside in that the useful signal, i.e. the communication signal, is massively impaired by pulse-shaped interference signals (e.g. the DME signals) because the power level of the OFDM-based communication signal is considerably smaller than the power level of the DME signals.
In a general manner, the above described problem can be formulated to the effect that, depending on the respective frequency band in which the OFDM-based system is operated, there will occur, at the receiver, pulse-shaped interferences from other systems which are operated in the same or the adjacent frequency band. These interferences, though being very short and thus impairing the OFDM signal only for a short time, mostly have a very high power and take a significant influence on the OFDM system.
Due to their high power, the interference pulses and, respectively, the samples of the OFDM signal affected by the interference are easily detectable. If the detected power has a sample value above a specific threshold value, the respective sample value is set to zero or its amplitude is reduced to a threshold value. The state of the art includes techniques provided to weaken pulse-interfering factors in communication or navigation systems. For this purpose, one will largely use the techniques of “pulse blanking” or “clipping” (amplitude limitation) or combinations of both techniques (see e.g. [2] and [3]).
Both in pulse blanking and in clipping, the influence of the pulse-shaped interference is considerably reduced, thus effecting an improvement of the overall performance of the OFDM system. However, the process will have an influence not only on the interference signal but also on the desired OFDM signal; this will entail losses in performance, causing a distinct reduction of the benefits obtained by pulse blanking or clipping.
In literature, it has already been discussed in [4], in the context of impulsive noise, what kinds of effects the technique of pulse blanking has on the performance of OFDM receivers and in what manner such influences can be reduced by optimizing the threshold value for pulse blanking in the OFDM receiver.
Further, in US-A-2006/0198453, US-A-2005/0220001, US-A-2006/0116095 and [5], techniques for compensating the impact of pulse blanking onto the desired OFDM signal have been proposed. All four techniques have in common that they subtract a compensation signal from the received frequency domain signal. In the first three references, the compensation signal is generated in the time domain with the objective to estimate ICI and those parts of the desired OFDM signal that have been blanked in the time domain received signal. However, the techniques according to the first three references are not targeted to directly reconstruct ICI in the frequency domain by exploiting known characteristics of OFDM signals in general and of the applied pulse blanking in particular. The technique according to [5] aims at estimating and subtracting the impulsive noise signal itself.